NERVOUS SYSTEM
The nervous system is composed of
specialized cells called neuron, which
exercise control by sending electrical
signals called nerve impulses. Nerve
cells are the structural and functional
unit of nervous system.
STRUCTURE OF NEURON
A neuron consists of three prominent parts1.Cell body- The cell body of neuron is also
called cyton or soma. It receives nerve
impulses from dendrites and transmit them to
axon.
2.Dendrites- dendrites are the parts of neuron
where sensation acquired. The information then
travels as an electric impulse towards the cell
body.
Axon:-It is a single, cylindrical
protoplasmic nerve fibre. At its
terminal axon is highly branched. The
axon conducts nerve impulses away
from the cell body.
ARRANGEMENT OF NEURON
The neurons lie end to end in chains to transmit
nerve impulses in the animal body. The neurons
are not connected. There occurs a very minute
gap between terminal portion of axon of one
neuron and the dendron of other neuron. This
minute gap is called synapse.
Axon terminal expand to form presynaptic knob
and dendrite forms post synaptic depression. In
between the two lies a fluid filled space called
synaptic cleft.
As the nerve impulses reaches the pre-synaptic
knob, the synaptic vesicles get stimulated to
release a chemical called neurotransmitter in
the synaptic cleft. The neurotransmitter
molecules diffuse across the gap to come in
contact with the chemoreceptor site in the
nerve impulses passes from one neuron to
other.
REFLEX ACTIONS
A Reflex actions may be defined as a
spontaneous, automatic and mechanical
response to a stimulus , acting on a specific
receptor , without the will of an animal . In
certain situation , sensation requires immediate
response as time used for processing may cause
harm to the body. In such situation reflex action
occur.
Example:-blinking of eyes ,withdraw of hand
suddenly pinched or touched by a hot object.
HUMAN NERVOUS SYSTEM
Human nervous system is the most complex.
It is divided into two main partsi)
Central nervous system
ii)
Peripheral nervous system
CENTRAL NERVOUS SYSTEM
It is hollow and lies on the mid-dorsal
part along the main axis of the body. It is
covered by a part of axial skeleton. The
CNS in turn consists of two parts1. Brain or Encephalon situated in the head.
2. Spinal cord or Myelon located in the neck
and trunk.
BRAIN
The brain is the widest and the uppermost part of
the central nervous system . It is the highest
coordinating centre in the body. Brain is situated
in the cranial cavity of the skull. The brain is soft,
whitish organ. It weighs 1.2 to 1.4 kg and forms
about 98% of the weight of the whole nervous
system. It has about 100 bilion neurons. Brain is
surrounded by three membranes called meninges
which provide protection to it. The space between
the meninges is filled with the cerebro- spinal
fluid .
Brain is divisible into three main region –
1.
FOREBRAIN- It forms the greater part of
the brain. It consists of three parts: olfactory
lobes, cerebral hemisphere and diencephalon.
i) Olfactory lobes- A pair of widely separated
club- shaped small structure . Each olfactory
lobe consist of an anterior olfactory bulb and
a posterior narrow olfactory stalk. These
lobes receive impulses from olfactoreceptors
and relay sense of smell to the temporal
region of the cerebrum.
(ii) Cerebral hemisphere- It is the largest part of the
brain . The two cerebral hemisphere lie side by side
separated by deep cerebral fissure. The cerebral
hemisphere divided into four lobes –
a. Frontal lobe is the region for speech ,facial,
muscular activities and mental activities.
b. Temporal lobe is the region for auditory reception
(hearing ) .
c. Occipital lobe is the region for visual reception
(sight )
d. Parietal lobe is the region for touch ,taste ,smell,
temperature and conscious association.
(iii) Diencephalon:-It lies on the side of cerebrum It
roof is called epithalamus, sides are called thalami
and its floor is called hypothalamus. Diencephalon
has a narrow cavity called third ventricle.
2. MID BRAIN:-It is significantly small region. It
consists of two fibre tracts called crura cerebri and
a pair of two swellings called superior and infeior
colliculi on each side. the two superior colliculi
have centres for sight reflexes and the two inferior
colliculi have centre for sight reflexes. It controls
the movement of head, neck, eye muscles ;changes
in pupil size as well as shape of eye lens.
3. HIND BRAIN:- The hind brain consists of
three parts- cerebellum, pons varolii and
medulla oblongata. The cerbellum is the second
largest part of the brain , constituting nearly
12% of it. It has two large, lateral cerebellar
hemispheres and a central vermis. Cerebellum
maintains the posture, equilibrium and muscles
tone. Pons varolii controls some aspects of
respiration. Medulla oblongata is the posterior
part of the brain. It controls rate of heartbeat,
breathing movements etc.
SPINAL CORD
Spinal cord is a cylindrical structure and is about
45cm long . It begins in continuation with the
medulla oblongata of brain extend downward up
to early part of the lumber region . It then extend to
the end of vertebral column as fibrous connective
called filum terminale. Internally the spinal cord
possess a narrow, fluid filled cavity called central
canal. Spinal cord is enclosed in the vertebral
column or backbone which protect it. It conducts
sensory and motor impulses to and from the brain.
It act as center for the reflex action
PERIPHERAL NERVOUS SYSTEM
It connects CNS with different part of the
body . It has two component, voluntary and
Autonomic nervous system. These are1. Voluntary peripheral nervous system is
under the control of will. It consist of nerves
that arise directly from CNS connecting
different body parts for voluntary control of
the brain.
2. Autonomic nervous system (involuntary
nervous system), on other hand, is not under
the control of human will. It developed
branches of some cranial and some spinal
nerves called visceral nerves. It is subdivided
into two parts:
(i) sympathetic nervous system
(ii) parasympathetic nervous system
The Organization of the
Sympathetic Division of the ANS
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Sympathetic ganglia
Sympathetic chain ganglia (paravertebral
ganglia) – preganglionic fibers of the
sympathetic NS that carry motor impulses to the
body wall or thoracic cavity synapses in chain
ganglia
Collateral ganglia (prevertebral ganglia) –
group of second order neurons that innervate
organs in the abdominopelvic region
Sympathetic Pathways
Sympathetic Pathways
Sympathetic Pathways
The Distribution of Sympathetic
Innervation
Animation: The sympathetic division (see tutorial)
Postganglionic fibers
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Rejoin spinal nerves and reach their destination
by way of the dorsal and ventral rami
Those targeting structures in the thoracic cavity
form sympathetic nerves
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Go directly to their destination
Abdominopelvic viscera
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Sympathetic innervation via preganglionic fibers
that synapse within collateral ganglia
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Splanchic nerves – carry fibers that synapse in
collatheral ganglia
Abdominopelvic viscera
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Celiac ganglion
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Superior mesenteric ganglion
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Innervates stomach, liver, gall bladder, pancreas,
spleen
Innervates small intestine and initial portion of large
intestine
Inferior mesenteric ganglion
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Innervates kidney, urinary bladder, sex organs, and
final portion of large intestine
Sympathetic activation
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Sympathetic activation is controlled by
sypathetic centers in the hypothalamus.
In crises, the entire sympathetic division
responds
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Sympathetic activation
Affects include increased alertness, energy and
euphoria, increased cardiovascular and respiratory
activities, elevation in muscle tone, mobilization of
energy resources
Neurotransmitters and
sympathetic function
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Stimulation of sympathetic division has two
distinct results
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Release of ACh or NE at specific locations
Secretion of E and NE into general circulation
Most postganglionic fibers are adrenergic, a few
are cholinergic or nitroxidergic
Sympathetic Variosities
Parasympathetic division
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Preganglionic neurons in the brainstem and
sacral segments of spinal cord
Ganglionic neurons in peripheral ganglia located
within or near target organs
The Organization of the
Parasympathetic Division of the
ANS
Organization and anatomy of the
parasympathetic division
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Preganglionic fibers of parasympathetic neurons
can be found in cranial nerves III, VI, IX, X
Sacral neurons form the pelvic nerves
Almost 75% of all parasympathetic outflow
travels along the vagus nerve (cranial nerve X)
The Distribution of
Parasympathetic Innervation
Parasympathetic activation

Effects produced by the parasympathetic division
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relaxation
food processing
energy absorption
Neurotransmitters and
parasympathetic functions
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All parasympathetic fibers release ACh
Short-lived response as ACH is broken down by
AChE and tissue cholinesterase
Sympathetic and
parasympathetic divisions

Sympathetic
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Parasympathetic
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•
Widespread influence on visceral and somatic
structures
Innervates only visceral structures serviced by cranial
nerves or lying within the abdominopelvic cavity
Effects produced by the parasympathetic branch
include increased secretion by digestive glands
Dual innervation = organs that receive input from
both systems
Anatomy
of
dual
innervation
 Sympathetic and parasympathetic systems
intermingle to form autonomic plexuses
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Cardiac plexus – sympathetic and parasympathetic
fibers bound for the heart and kungs pass through the
cardiac plexus
Pulmonary plexus
Esophageal plexus
Celiac plexus
Inferior mesenteric plexus
Hypogastric plexus
The Autonomic Plexuses
Comparison of the two divisions

Important physiological and functional
differences exist
Summary: The Anatomical
Differences between the
Sympathetic and Parasympathetic
Divisions
A Comparison of Somatic and
Autonomic Function
Higher levels of autonomic
control

Activity in the ANS is controlled by centers in
the brainstem that deal with visceral functioning
Levels of Autonomic Control
Example of higher-level of
autonomic function would
be increased heart rate
when you see a person that
you dislike.
Higher order functions
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Are performed by the cerebral cortex and involve complex
interactions
Involve conscious and unconscious information processing
Are subject to modification and adjustment over time
Memory
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Short term or long term
Memory consolidation is moving from short term to long term
 Hippocampus is essential for memory consolidation
Mechanisms involved in memory formation and storage are:
 Increased release of neurotransmitter
 Formation of additional synaptic connection
 Formation of memory engrams (single circuit that
correspond to single memory)
Amnesia is the loss of memory due to disease or trauma
Memory
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Memory that can be voluntarily retrieved and
verbally expressed are called declarative
memories
Conversion of a short term memory to a long
term memory is called memory consolidation
Memory Storage
Consciousness

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Deep sleep, the body relaxes and cerebral cortex activity is low
The reticular activating system (RAS) is important to arousal
and maintenance of consciousness
 RAS is located in the mesencephalon
The Reticular Activating System
Age-related changes
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Reduction in brain size and weight
Reduction in the number of neurons
Decrease in blood flow to the brain
Changes in synaptic organization of the brain
Intracellular and extracellular changes in CNS neurons
You should now be familiar with:
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The organization of the autonomic nervous system.
The structures and functions of the sympathetic and
parasympathetic divisions of the ANS.
The mechanisms of neurotransmitter release in the
sympathetic and parasympathetic divisions.
The effects of sympathetic and parasympathetic
neurotransmitters on target organs and tissues.
The hierarchy of interacting levels of control in the ANS.
How memories are created, stored and recalled.
The effects of aging on the nervous system.